Temperature detection has become a significant measurement in product and process quality control. Radiation detectors are particularly useful because they provide a non-contact measurement which allows continuous measurement of an ongoing process such as product moving along a conveyor belt. Radiation sensors also provide a relatively fast response in the order of 100 milliseconds.
Temperature detection systems compare detected radiation with a reference. A difficulty arises where the background radiation in the system is not constant, thus requiring frequent recalibration of the system. For example, the radiation detector may view a continuous or intermittent bead of hot adhesive which has just been laid on a package to confirm that the adhesive is being laid properly. In a hot environment, the adhesive may only be a few degrees higher in temperature than the surrounding packaging, so a change in environmental temperature of only one or two degrees may place the detector out of calibration.
A solution to the problem of changing background temperature was presented in U.S. Pat. No. 4,831,258. In that system, two radiation sensors are provided to view adjacent target surfaces and provide a differential output. Accordingly, the reference level to which the principal radiation sensor is compared is that of the radiation from the surrounding environment. In a typical application, the principal radiation sensor would view a continuous or intermittent bead of adhesive while the reference sensor views an adjacent surface, the two sensors being positioned in line perpendicular to the direction of movement of the product being monitored.
Although the differential approach has been very successful, the 100 millisecond time constant of thermal radiation sensors, those such as thermopiles which convert radiation to thermal energy and thermal energy to electrical output, has limited their application to systems where the hot target to be sensed is very long or where the conveyor moves very slowly. Due to thermal time constants, the response of a thermal radiation sensor begins to fall at about 1.5 hertz. At higher frequencies more complex and expensive quantum photoelectric systems, which convert radiation directly to electrical output, have been required.